Tank is fairly new, but only partly. Upgraded from a 30 to a 75. Started noticing the "slime" a few weeks prior to making the switch over. The only change in the 30 that I can think of was that I was running skimmer less, so maybe there was a build up of some type? I was dosing carbon, purigen, and aqua vitro remediation. I thought maybe once I got more water volume, more water flow, and a little more stout filtration things might clear up, but they have not.